1. Field of the Invention
The present invention relates to a method for measuring concave-convex (hereinafter, “concave-convex” is referred to read as “irregularity”) exhibited on a surface of a body of rotation such as a tire and especially relates to correction of data of irregularity taken successively at every equal angle around an axis of rotation.
2. Description of the Related Art
Conventionally, as one of external appearance inspection on a finished tire carried out at the time of shipping produced tires, inspections of condition of irregularities exhibited on a tire side portion or on a tire crown portion has been performed. (For example, make reference to patent documents 1 and 2).
FIG. 4 shows an example of constitution of an apparatus 50 for inspecting irregularities exhibited on a tire side portion, and this inspection apparatus 50 is operated such that after mounting the tire 10 to be inspected on the limb 52n of the tire holding device 52 coupled to the rotary driving device 51, the tire is filled to a predetermined internal pressure and the distance between the surface of the tire side portion 10S of the tire 10 and the non-contact displacement probe 53, which is positioned in vicinity of the tire side portion 10S and provided with LED and laser, is measured at a predetermined sampling interval while the tire is being rotated at a constant rotational speed. The non-contact displacement probe 53 measures, with non-contacting, the distance from this probe 53 to the tire side portion 10S by receiving an inspection light irradiated to the tire side portion 10S and reflected therefrom. Thereby condition of irregularity exhibited on the tire side 10S is measured and the measured distance data is forwarded to the control and processor unit 54 and converted to an irregularity data of the tire side 10S.
Also, the irregularity inspection apparatus 50 has the 2nd non-contacted displacement probe 55, which is arranged at the position facing the tire crown portion 10t. Though, in the irregularity inspection apparatus 50, the diameter of the tire is measured by measuring the distance between 2nd non-contacted displacement probe 55 and the tire crown portion 10t, it is possible to measure the condition of irregularity exhibited on the surface of the tire crown portion 10t as well as the tire side portion 10S by utilizing the 2nd non-contact displacement probe 55.
In case of measuring the irregularity on the surface of the tire crown portion 10t having the tire tread pattern such as a block pattern by means of the non-contact displacement probe 55, as shown in FIG. 5, the radius value rk presenting the irregularity value rk at the k th measurement point on the surface of the tire crown portion 10t can be given by the relation, rk=L0−Lk wherein Lk denotes the above distance data and L0 denotes the distance between the center of rotation of the tire 10 and the sensor 14. Accordingly, upon storing design value of the radius value Rk0 at each of measurement points Xk (k=1˜N) on the tire 10 and the above L0 as constant values in advance, the difference value Δrk between the radius value rk at each measurement point Xk on the tire crown portion 10t and the above design value Rk0 can be computed. Then, when the value of Δrk exceeds a predetermined value, existence of abnormality on irregularity is considered. Therefore, by measuring the radius value rk Of the tire crown portion 10t the abnormality of the irregularity exhibited on the tire crown portion 10t such as irregularity on the pattern block of the contact patch can be detected.    [Patent Document 1]    Japanese laid open publication document No. 2004-361344    [Patent Document 2]    Japanese laid open publication document No. 2005-501245
However, according to the foregoing method, in the case where the tire 10 is arranged eccentrically with the center of rotation of the inspection apparatus which is the rotation axis of the rotary driving device 51, the measurement points Xk can not be placed at equal intervals as shown by FIGS. 6 (a) and (b). In other words, as depicted by A˜B, D˜A in the same drawing, the surface located at the portion near the center of rotation of the inspection apparatus swells and the one located at the portion for away therefrom as depicted by B˜D shrinks resulting in giving rise to a problem of the measurement data including the surface swelled or shrinked affected by the existence of the eccentricity.
Since it was difficult to grasp the amount of eccentricity accurately, currently it is obliged to accept the result of measurement in which the condition of eccentricity is involved. Accordingly, when a comparison with a normal tire utilized as a standard as to the irregularity of surface, it is impossible to distinguish the cause of discrepancy on the irregularity whether it is resulted from the defectiveness of the tire or from the eccentricity, and such a condition has been an obstacle for an automatic inspection of irregularity of surface of the tire.
In this instance, it is considered to overcome the difficulty due to the eccentricity by modifying the constitution (hardware) of the inspection apparatus, it is a cost consuming task and further when a amount of eccentricity can not be grasped accurately, it is hardly possible to remove effect caused by the eccentricity completely.
The present invention has been made to cope with the problem in the related art and the object thereof is to provide a method for correcting the data of the irregularity exhibited on the surface of a body of rotation, in which difficulty of eccentricity is involved, into the data devoid of effect due to the eccentricity.